Real Madrid could face disqualification from the Copa del Rey after appearing to field an ineligible player in their 3-1 fourth round first leg win over Cadiz.
Denis Cheryshev had given Rafael Benitez's side the lead at the Ramon de Carranza but the Russia international should have been suspended after picking up three yellow cards with former club Villarreal last season.
Last season Osasuna were kicked out of the tournament for playing an ineligible player while Benitez was in charge of Valencia when they were disqualified from the Copa del Rey having played too many non-EU players in a tie in 2001.
Cheryshev's early strike was followed up by two from Isco before Kike Marquez grabbed a late consolation for their third tier opponents.
There was no such scare for Barcelona who breezed to a 6-1 win over Segunda B Villanovense and booked their place in the last 16 having drawn their first leg 0-0 at the end of last month.
Barca - whose schedule was brought forward to accomodate their trip to the World Club Championship - fielded a virtual reserve team but a hat-trick from Sandro Ramirez, two goals from Munir and one from Dani Alves saw them home.
Gary Neville was given an early glimpse of the size of the task facing him at Valencia after his new side were forced to come from behind to beat third-tier Barakaldo.
Despite being appointed on Wednesday, Neville will not take charge of the team until next Sunday, leaving brother Phil to assist caretaker-boss Voro in the last-32, first-leg fixture.
Alain Arroyo stunned the visitors by netting the opener after 16 minutes but Valencia avoided embarrassment as goals from Joao Cancelo, Jose Gaya and a Dani Parejo penalty secured a 3-1 win.
Deportivo La Coruna were forced to come from behind to earn a 2-1 win away to Segunda Division strugglers Llagostera.
Juanjo Exposito gave the hosts a surprise 22nd-minute lead before Federico Cartabia equalised on the hour mark. Despite the subsequent sending-off of Luisinho for a second bookable offence, Deportivo grabbed victory through Oriol Riera 14 minutes from time.
Goals from Coke, Michael Krohn-Dehli and Ciro Immobile gave Sevilla a 3-0 win away to UD Logrones, while two for Iago Aspas and a third from John Guidetti saw Celta VIgo win 3-1 in Almeria.
In two all-Primera Division ties, Sporting Gijon slumped to their fourth straight defeat as goals from Alvaro Vadillo and Juan Vargas gave Real Betis a 2-0 first-leg win in Seville, while Rayo Vallecano beat Getafe by the same score.
